There's a whole subculture out there of converted vans. I'm in the process of converting a Transit mid-roof (I can stand upright; Ed at 6'3" has to duck a little) into a minimalist camper. But once you discover the forums, you'll also find people who have used units for sale, too. My favorite hangout is the www.fordtransitusaforum.com - there's an entire section devoted to conversions. The only problem you might find with a used conversion is that people build them to fit specific needs (stuff that a factory-built B can't do) like hauling mountain bikes or surfboards, etc.
We wanted to build our own, because we DON'T need many of the amenities offered as "standard" on the pricey Class B's - and we will still need to utilize our van as a cargo van on occasion.
There's also companies like Sportsmobile, for example, that will build to your specs
On edit: despite the dour warnings posted about lack of height and space - getting MPGs in the mid-teens and being able to negotiate narrow twisty streets and park just about anywhere, are some of the advantages to being compact ;-) And in MY case, my "cargo van" is allowed to be parked in our condo driveway. An RV has 24 hours allowed for loading/unloading. And the total cost of our converted, brand new (2015) van will be less than $40k